In order to investigate the possibility of pathogenic Escherichia coli penetrating the bloodstream via the intestinal mucosa in normal and stressed turkeys and chickens, birds were inoculated orally with the bacteria or exposed environmentally to it. Immediately after hatch, intestines contained a substantial number of coliform bacteria that increased with time. In orally infected turkeys, the pathogenic bacteria (nalidixic-acid-resistant O78) replaced 10%-50% of the native coliform flora but could not be isolated from the trachea or blood. Environmentally exposed groups exhibited pathogenic bacteria in intestines but not in blood. Stressing of exposed turkeys resulted in isolation of the pathogenic bacteria from blood and even spleen. In orally infected broiler chickens, stress resulted in bacteremia and mortality. Chickens that were exposed to pathogenic bacteria at a young age and showed no mortality or morbidity demonstrated no detrimental effects due to challenge with the same pathogenic bacteria later in life. Stress seems to cause penetration of the pathogenic bacteria into the bloodstream, which in turn can cause severe disease and mortality.

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An enzyme-linked immunosorbent assay (ELISA) was modified for detection of antibodies against the two main pathogenic serotypes of Escherichia coli: serotypes O78:K80 and O2:K1. The ELISA was a more sensitive and repeatable test than the indirect hemagglutination test (IHT), which is a common method for detecting antibodies against E. coli. Cross-reactivity between the two strains was measured by reacting antisera of each serotype against homologous and heterologous antigens. The results suggest that aside from similar determinants expressed by the two serotypes, serotype O2:K1 expresses more strain-specific determinants than does O78:K80. Comparison of mean antibody titers of immunized chicks by IHT and ELISA along the primary response revealed that during the first 15 days after immunization with inactivated E. coli, the titers in both tests were parallel. After 15 days post-immunization, antibody titers measured by IHT decreased rapidly, whereas titers measured by ELISA decreased only slightly. In addition, a higher correlation was found between titers detected by ELISA and survival through challenge with E. coli than between titers detected with IHT and survival through challenge. The results suggest that the ELISA is a better test for detection of antibody in flocks suspected of being infected with E. coli.

Ultrasonic inactivation of Escherichia coli followed by irradiation was found to be the most efficient method for preparation of an effective vaccine against colibacillosis. Challenge experiments revealed that this vaccine provided the best protection compared with other methods of inactivation: heat, formaldehyde, and irradiation. Preparing the ultrasonicated vaccine from O2:K1 strain increased its range and also supported adequate protection against homologous strain O78:K80. The degree of protection conferred by the vaccine was positively correlated with the antibody titer against E. coli as measured on day of challenge. Low antibody titers detected 5 days post-vaccination resulted in only 20% protection. High antibody titers detected at 8 and 15 days post-vaccination correlated with a low number of chicks with lesions. In each challenged group, the live chicks that did not develop lesions had higher antibody titers than chicks with lesions, revealing a correlation between numbers of chicks with lesions and antibody titers as measured by enzyme-linked immunosorbent assay.

SUMMARYA non-pathogenic, piliated strain of Escherichia coli (BT-7;Frommer et al., 1990), isolated from a meat-type chicken flock, was studied as a candidate for a live vaccine to protect chickens from E. coli infection. Active immunization provided substantial protection of chicks vaccinated at 14 or 21 days of age, resulting in better resistance to challenge than in those vaccinated at 1 or 7 days. Chicks vaccinated at 21 days of age and challenged 1 week later with pathogenic E. coli strains 01-.K1, 02:K1 or 078:K80, exhibited good protection for at least 2 weeks against all strains. Three vaccination routes were found to give the highest resistance to challenge with pathogenic E. coli strain 078:K80. Intramuscular (i.m.) at 7 and 21 days of age, i.m. at 21 days of age and spray at 7, followed by per os at 21 days of age. Vaccination per os once at 7 or twice at 7 and 21 days resulted in good protection. Chicks exhibiting high antibody titres by ELISA were well-protected against challenge.